There is some evidence that individuals with Down syndrome (DS) may have a poorer
mathematical performance and a poorer working memory (WM) than typically
developing (TD) children of the same mental age. In both typical and atypical individuals,
different aspects of arithmetic and their relationships with WM have been largely studied,
but the specific contribution of WM to the representation and elaboration of non-symbolic
quantities has received little attention. The present study examined whether individuals
with DS are as capable as TD children matched for fluid intelligence of estimating
numerosity both of single sets and of added sets resulting when two sequentially
presented sets are added together, also considering how these tasks related to verbal and
visuospatial WM. Results showed that the DS group’s performance was significantly worse
than the TD group’s in numerosity estimation involving one set, but not when estimating
the numerosity resulting from the addition. Success in the addition task was related to
success in the working memory tasks, but only for the group with DS; this applied
especially to the visuospatial component, which (unlike the verbal component) was not
impaired in the group with DS. It is concluded that the two numerosity tasks involve
different processes. It is concluded that the arithmetical and working memory difficulties
of individuals with DS are not general, and they can draw on their WM resources when
estimating the numerosity of additions.
